The present invention relates to a mold adapted for use in injection molding of thermoplastic resin articles, and to an improved method of injection molding making use of the mold.
More particularly, the present invention is concerned with a novel construction of a hot-runner type mold which can be suitably used in simultaneous molding of a number of articles without being accompanied by any leakage of the molten resin or the clogging of the gate with the molten resin, and capable of functioning without fail and ensuring a good quality of the products through an adequate cooling and solidification of the resin, as well as with a method of carrying out an injection molding making use of the hot-runner type mold.
The hot-runner technic in the injection molding process is now attracting attentions of those who are concerned with this field of industry, as being one of the most effective measures for the rationalization of the whole molding process, because this technic can exclude needless and obstructive by-products or appendix such as solidified sprues and runners annexed to the molded products. In fact, many successful examples of practical use of this hot-runner technic have been reported.
However, this hot-runner technic cannot be advantageously used in the molding processes in which the ratio of weight of runner to the total weight of the product or products is relatively large, as in the production of small articles or in the simultaneous molding of a number of articles. Namely, in these cases, the hot-runner system for one molded article occupies a too large space, so that the number of articles obtainable from a given size of mold, in case of a simultaneous molding of a plurality of articles, is limited not by the size of the article but rather by the size of the runner system, often resulting in an unacceptably small yield. This goes quite contrary to the lowering of the production cost which is the major aim of the hot-runner technic.
In order to overcome this problem, there has been proposed a mold as shown in FIGS. 1 and 2, having a simple and, accordingly, less expensive construction and specifically intended for use in a simultaneous molding of small-sized articles by means of the hot-runner molding technic.
More specifically, referring to FIGS. 1 and 2, blocks 4 and 5 of a mold are fixed, respectively, to a stationary die plate 2 and a movable die plate 3 of an injection molding machine. A moving side cavity block 20 and a stationary-side cavity block 20a are connected to the blocks 5 and 4, respectively, in a predetermined positional relationship which will be detailed later. A hot runner block is constituted by a hot-runner block body 10 and a hot sprue bush 10a. The hot sprue bush 10a is fixed to the block 4 and is closely contacted at its one end by a nozzle 1 of the injection molding machine in such a manner that an injection port of the nozzle for injecting a molten resin is made to communicate a molten resin supply port 11 of the hot sprue bush.
The hot-runner block body 10 is a rectangular member the longer side of which extends in the direction normal to the plane of the drawing and has two parallel surfaces facing in opposite directions. These parallel surfaces constitute gate opening surfaces 15. A plurality of gates 13 are formed to open in each of the gate opening surfaces 15, 15, in a row extending along the length of the hot-runner block body 10.
The gates 13 are in communication with a main runner 12b extending in the longitudinal direction of the body 10, through branch runners 12c, while the main runner 12b communicates the molten resin supply port 11, through a sprue 12a.
Heating bodies 14 are embedded in the hot-runner block body 10 and in the hot sprue bush 10a, so as to maintain them at a temperature high enough to ensure a smooth flowing of the resin therethrough.
The moving-side cavity block 20 has two opposing contacting surfaces 25 which are adapted to contact the gate opening surfaces 15 of the hot-runner block body 10, at least when the mold is closed. The moving-side cavity block 20 further has a plurality of recesses formed in the parting surface 23, which constitutes molding cavities 21 for molding the destined articles. Each molding cavity 21 has a molten-resin pouring port 21a opening at portions of the cavity block 20 corresponding to each of the gates 13 formed in the hot-runner block body 10.
The stationary side cavity block 20a has two opposing contacting surfaces 25a adapted to slidingly contact the corresponding gate opening surfaces 15 of the hot-runner block body 10. A spring 6 is disposed between the stationary block 20a and the block 4, so as to bias them away from each other. The stroke over which these two members are movable away from each other is however limited by a stopper 7.
Conduits or passages for cooling water are formed in the cavity blocks 20 and 20a, so that these cavities may be cooled down to a temperature low enough to promote the solidification of the resin.
In FIG. 1, the mold is shown in the state of injection, in which the molten resin is injected at a high pressure into the mold through the nozzle of the injection molding machine, while, in FIG. 2, the mold is shown in the opened state in which the movable die plate 3 of the injection molding machine has been moved to allow an ejection of the molded articles 40.
During the opening of the mold, i.e. in the transition period in which the state of the mold is changed from the closed state as shown in FIG. 1 to the opened state as shown in FIG. 2, the spring 6 functions to move the stationary-side cavity block 20a in relation to the block 4. Consequently, the stationary-side cavity block 20a is moved relatively to the hot-runner block body 10, so that the contacting surfaces 25a comes to close the openings of the gates 13, thereby to prevent molten resin from leaking out of the gates. At the same time, the molded articles 40 are separated from the fluidized portion of the material resin.
On the other hand, due to the opening of the mold the movable die plate 3 is moved until it abuts against a base plate (not shown) of the injection molding machine. On that occasion, an ejecting rod 9 abuts against a protrusion (not shown) provided on said base plate, and is relatively moved in right direction in FIG. 2 with respect to the fixed block 5. Accordingly, the ejector pins 9b supported on a plate 9a connected with the ejecting rod 9 eject the molded articles 40 from the molding cavities 21.
In the hot-runner mold having the described arrangement, the construction of the gates is most simplified and the distance between adjacent gates, i.e. the pitch at which the gates are disposed, is minimized, because the independent installations of heat control means and leak-prevention valves for respective gates are eliminated. At the same time, since the gate openings are closed by the contacting surfaces of the cavity blocks as stated before, so that the troubles such as leaking out of the molten resin from the gate opening are fairly avoided.
This construction of the mold however has a fatal drawback. Namely, in this mold, the hot-runner block which has to be maintained at a high temperature is kept in contact with the cavity blocks which have to be maintained at a low temperature, over the whole period of the molding process. Consequently, the temperature of the gates which are located near the boundary between the hot and cold parts tends to drop below the desired temperature, at a period in which the gate temperature has to be sufficiently high, i.e. at the time of the injection. To the contrary, the cavity block, which has to be cold enough to promote the solidification of the resin, tends to become excessively hot.
In order to maintain a sufficiently high temperature of the gates, it has been proposed to further heat up the whole hot-runner block, or to provide a means for heating specifically the portions around the gates.
However, the former way of solution is unacceptable in that it is likely to incur an excessive heating of the portions of the resin passages resulting, possibly, in undesirable thermal decomposition of the resin.
The latter way of solution is also unacceptable in that the thermal flow toward the cavity blocks is enhanced to hinder an appropriate cooling of the articles, possibly resulting in various problems such as deteriorated dimensional precision, and buckling and deformation of the articles.